COMPARISON OF THE MAGNITUDE OF EROSION ALONG TWO LARGE REGULATED RIVERS1

ABSTRACT: Historical inventories of sand bar number and area are sufficient to detect large-scale differences in geomorphic adjustment among regulated rivers that flow through canyons with abundant debris fans. In these canyons, bedrock and large boulders create constrictions and expansions, and alluvial bars occur in associated eddies at predictable sites. Although these bars may fluctuate considerably in size, the locations of these bars rarely change, and their characteristics can be compared through time and among rivers. The area of sand bars exposed at low discharge in Hells Canyon has decreased 50 percent since dam closure, and most of the erosion occurred in the first nine years after dam closure. The number and size of sand bars in Grand Canyon downstream from Glen Canyon Dam have decreased much less; the number of sand bars decreased by 40 percent in some 8.3-km reaches, but by less than 20 percent elsewhere. These differences are in part related to the fact that flood regulation is much greater in Grand Canyon than in Hells Canyon, and that downstream tributaries resupply sediment to Grand Canyon but not to most of Hells Canyon.     

Boater preferences for beach characteristics downstream from Glen Canyon Dam, Arizona

Release flow decisions are increasingly being influenced by an array of social values, including those related to river-based recreation. A substantial portion of past recreation research on downstream impacts of dams has focused on variability of instream flows. This study complements past research by assessing user preferences for beach characteristics affected by long-term impacts of flow regimes. Based upon a study of three recreational user groups (private trip leaders, commercial passengers, and river guides) of the Colorado River in Grand Canyon, preferences for beach size, presence of shade on beach, and presence of vegetation on beach are examined. Results indicate that large size beaches with shade from trees are setting characteristics with highly reliable and strong user preferences. The multinomial regression models developed for each user group indicate that 80% of all respondents would choose beach campsites 800 m(2); results were the same regardless of respondents' past boating experience, boat type (i.e. oar or motorized), or group size. In addition, size of beach was consistently reported to be a trip feature of moderate importance to respondents' river trip. Implications of this research are related to future prospects for controlled floods (i.e. spike flows) released from Glen Canyon Dam.     

Environmental management of the Colorado River within the Grand Canyon

Irreversible environmental changes are occurring along the Colorado River in the Grand Canyon as a result of regulation of the river flow by the Glen Canyon Dam. The questions of primary importance in managing this great natural resource are 1) in what manner and how rapidly are the physical and ecological adjustments taking place, and 2) is the increased use of the river for recreational boating contributing to the degradation? Human use along the Colorado River is limited, for the most part, to the relic, pre-dam fluvial deposits colloquially called “beaches.” With the new river regime these deposits are positioned well above the present high-water stage, 27,000 cubic feet/second (cfs), or 765 cubic meters/second (cms), so they are not replenished periodically as they were prior to construction of the dam in 1963. The dominant natural processes now are aeolian sand transport and mass wasting. The float-trip passengers use the river beaches for hiking, camping, and. lunch stops. At the most desirable sites thirty to forty people camp on the beaches each night over a four to five month season. Human impact includes incorporation of campsite litter, burial of chemically treated waste, and the direct stress associated with people walking on the vegetation and unstable sedimentary deposits. Results of our investigations indicate that the rate of degradation at the most heavily used sites exceeds the capacity of aeolian processes to reestablish natural landscapes. Therefore, careful management of float trjps is needed if these environments are to be maintained in a natural state rather than a “sand-box” state.     

Simulating the effects of Glen Canyon Dam releases on Grand Canyon river trips

The Shechter-Lucas wilderness use simulation model (WUSM) has been modified to quantify the effects of fluctuating Glen Canyon Dam releases on Grand Canyon river trips. The model now simulates changes in flow as predicted by the Streamflow Synthesis and Reservoir Regulation computer model for the Colorado River. This revised WUSM provides data on several flow-related effects, including delays at rapids, encounters with other parties, and the time available for visiting attraction sites. Nine flow alternatives are evaluated and compared. Analysis of these outputs provides useful information to the Bureau of Reclamation on how the operation of Glen Canyon Dam effects river trips through Grand Canyon National Park.     

Environmental Impact Assessment of Sand Mining from the Small Catchment Rivers in the Southwestern Coast of India: A Case Study

In the past few decades, the demand for construction grade sand is increasing in many parts of the world due to rapid economic development and subsequent growth of building activities. This, in many of the occasions, has resulted in indiscriminate mining of sand from instream and floodplain areas leading to severe damages to the river basin environment. The case is rather alarming in the small catchment rivers like those draining the southwestern coast of India due to limited sand resources in their alluvial reaches. Moreover, lack of adequate information on the environmental impact of river sand mining is a major lacuna challenging regulatory efforts in many developing countries. Therefore, a scientific assessment is a pre-requisite in formulating management strategies in the sand mining-hit areas. In this context, a study has been made as a case to address the environmental impact of sand mining from the instream and floodplain areas of three important rivers in the southwestern coast of India namely the Chalakudy, Periyar and Muvattupuzha rivers, whose lowlands host one of the fast developing urban-cum-industrial centre, the Kochi city. The study reveals that an amount of 11.527 million ty⁻¹ of sand (8.764 million ty⁻¹ of instream sand and 2.763 million ty⁻¹ of floodplain sand) is being mined from the midland and lowland reaches of these rivers for construction of buildings and other infrastructural facilities in Kochi city and its satellite townships. Environmental Impact Assessment (EIA) carried out as a part of this investigation shows that the activities associated with mining and processing of sands have not only affected the health of the river ecosystems but also degraded its overbank areas to a large extent. Considering the degree of degradation caused by sand mining from these rivers, no mining scenario may be opted in the deeper zones of the river channels. Also, a set of suggestions are made for the overall improvement of the rivers and its biophysical environment.     

External threats: the dilemma of resource management on the Colorado River in Grand Canyon National park,USA

The United States Congress established Grand Canyon National Park in 1919 to preserve for posterity the outstanding natural attributes of the canyon cut by the Colorado River. In some cases National Park Service attempts to maintain Grand Canyon's natural environment have been thwarted by activities outside the park. One of the most obvious external threats is Glen Canyon Dam, only 26 km upstream from the park boundary. Constructed in 1963, this gigantic dam has greatly altered the physicochemical and biological characteristics of 446 km of the Colorado River in Grand Canyon National Park. The river's aquatic ecosystem has been greatly modified through the loss of indigenous species and the addition of numerous exotics. We consider this anexotic ecosystem. The riparian ecosystem has been less modified, with addition of a few exotics and no loss of natives—this we consider anaturalized ecosystem.The great dilemma now faced by park managers is that, after 20 years of managing resources along a river controlled by Glen Canyon Dam, the Bureau of Reclamation has proposed major changes in operational procedures for the dam. Scientists and managers from the National Park Service, Bureau of Reclamation, and cooperating federal and state resource management agencies are using a systems analysis approach to examine the impacts of various Colorado River flow regimes on aquatic, riparian, and recreational parameters in the park. This approach will help in the development of management alternatives designed to permit the most efficient use of that river's natural resources without their destruction.     

Recreational impacts on Colorado River beaches in Glen Canyon,Arizona

Recreational impact was measured on eight beaches in Glen Canyon National Recreation Area and 15 beaches in Grand Canyon National Park using permanently located transects and plots. Recreational impact indices included densities of human trash and charcoal and a measure of sand discoloration due to charcoal. Significant increases in the indices occurred on several Glen Canyon beaches over a seven-month period. Sand discoloration became significantly higher over all Glen Canyon beaches during the same time period. All indices were significantly higher in Glen Canyon than on similar Grand Canyon beaches. These differences are probably due to differences in: (a) level of impacts tolerated by the respective management regimes and, (b) in the number of user days among the two National Park Service administrative units. Management alternatives are presented for reversing the present trends of recreational impact on Glen Canyon beaches.     

Regional economic impacts of Grand Canyon river runners

Economic impact analysis (EIA) of outdoor recreation can provide critical social information concerning the utilization of natural resources. Outdoor recreation and other non-consumptive uses of resources are viewed as environmentally friendly alternatives to extractive-type industries. While outdoor recreation can be an appropriate use of resources, it generates both beneficial and adverse socioeconomic impacts on rural communities. The authors used EIA to assess the regional economic impacts of rafting in Grand Canyon National Park. The Grand Canyon region of northern Arizona represents a rural US economy that is highly dependent upon tourism and recreational expenditures. The purpose of this research is twofold. The first is to ascertain the previously unknown regional economic impacts of Grand Canyon river runners. The second purpose is to examine attributes of these economic impacts in terms of regional multipliers, leakage, and types of employment created. Most of the literature on economic impacts of outdoor recreation has focused strictly on the positive economic impacts, failing to illuminate the coinciding adverse and constraining economic impacts. Examining the attributes of economic impacts can highlight deficiencies and constraints that limit the economic benefits of recreation and tourism. Regional expenditure information was obtained by surveying non-commercial boaters and commercial outfitters. The authors used IMPLAN input-output modeling to assess direct, indirect, and induced effects of Grand Canyon river runners. Multipliers were calculated for output, employment, and income. Over 22,000 people rafted on the Colorado River through Grand Canyon National Park in 2001, resulting in an estimated $21,100,000 of regional expenditures to the greater Grand Canyon economy. However, over 50% of all rafting-related expenditures were not captured by the regional economy and many of the jobs created by the rafting industry are lower-wage and seasonal. Policy recommendations are given for increasing the regional retention of rafting expenditures and for understanding both the beneficial and adverse impacts that accompany outdoor recreation in rural areas.     

Analysis of bank erosion on the Merced River,Yosemite Valley,Yosemite National Park,California, USA

Channel changes from 1919 to 1989 were documented in two study reaches of the Merced River in Yosemite National Park through a review of historical photographs and documents and a comparison of survey data. Bank erosion was prevalent and channel width increased an average of 27% in the upstream reach, where human use was concentrated. Here, trampling of the banks and riparian vegetation was common, and banks eroded on straight stretches as frequently as on meander bends. Six bridges in the upper reach constrict the channel by an average of 38% of the original width, causing severe erosion. In the downstream control reach, where human use was minimal, channel widths both decreased and increased, with a mean increase of only 4% since 1919. Bank erosion in the control reach occurred primarily on meander bends. The control reach also had denser stands of riparian vegetation and a higher frequency of large woody debris in channels. There is only one bridge in the lower reach, located at the downstream end. Since 1919, bank erosion in the impacted upstream reach contributed a significant amount of sediment (74,800 tonnes, equivalent to 2.0 t/km²/yr) to the river. An analysis of 75 years of precipitation and hydrologic records showed no trends responsible for bank erosion in the upper reach. Sediment input to the upper reach has not changed significantly during the study period. Floodplain soils are sandy, with low cohesion and are easily detached by lateral erosion. The degree of channel widening was positively correlated with the percentage of bare ground on the streambanks and low bank stability ratings. Low bank stability ratings were, in turn, strongly associated with high human use areas. Channel widening and bank erosion in the upper reach were due primarily to destruction of riparian vegetation by human trampling and the effect of bridge constrictions on high flow, and secondarily to poorly installed channel revetments. Several specific recommendations for river restoration were provided to park management.     

A survey of trampling effects on vegetation and soil in eight tropical and subtropical sites

Impacts of recreation, especially of vehicles and walkers, were studied in eight tropical or subtropical public sites in Queensland. In each site, plant species number, vegetation cover, plant height, and species cover and frequency in untrampled, slightly trampled, moderately trampled, and heavily trampled areas were counted or measured. Soil penetration resistance and soil organic matter were also recorded. In two of these eight sites, plant cover, height, leaf length, leaf width, and leaf thickness of each species were measured. Some species of grass such asCynodon dactylon were present in areas subject to all degrees of trampling impact and some tussock species, particularlyEragrostis tenuifolia andSporobolus elongatus, were only present in trampled areas. Woody plants occurred only on untrampled areas. The number of species and all the vegetative measurements mentioned above were reduced as wear increased. Plant height was reduced dramatically by even light trampling. Tall plants appeared to be more sensitive to trampling than short plants. No clear relationship between soil organic matter content and trampling intensity was found.     

GEOMORPHIC AND PEDOLOGIC INFLUENCE ON SMALL-SCALE EPHEMERAL CHANNEL DIMENSION IN RANGELANDS1

ABSTRACT: Geomorphic processes may partly determine channel geometry. Soil particle uplift during freezing and thawing cycles and bank sloughing during wetting and drying periods were observed. Soil properties and channel dimension were measured to determine the dominant processes controlling channel geometry in eight small (mean area 0.096 km²) drainages in Logan Canyon, Utah. Soil cohesion was low (plasticity index > 15) for all but one of the drainages sampled. Basin scale geomorphic variables were examined to determine if they control channel dimension. Bankfull width was highly correlated to channel length and valley length with r² values of 0.85 and 0.84, respectively. A strong canonical correlation (0.64) showed that distance from the watershed divide, bank liquid limit, and bank sand content were effective predictor variables of bankfull width and depth. The interrelations between geomorphic and pedogenic processes were the strongest determinants of ephemeral channel dimension in this study.     

Plant morphological characteristics and resistance to simulated trampling

The relationship between responses of plants to trampling and their morphological characteristics was studied in a glasshouse experiment. Thirteen species with four different growth forms were used in this experiment. They were five tussock species.Chloris gayana, Eragrostis tenuifolia, Lolium perenne, Panicum maximum, andSporobolus elongatus; three prostate grasses,Axonopus compressus, Cynodon dactylon, andTrifolium repens, two herbaceous species,Daucus glochidiatus andHypochoeris radicata; and three woody species,Acacia macradenia, Acrotriche aggregata, andSida rhombifolia. These species were subjected to three levels of simulated trampling. For each species, measurements were taken of aboveground biomass, root biomass, leaf length, leaf width, leaf thickness, leaf number, broken leaf number and plant height. Overall, these measurements were greatest in the control plants, moderate in the level of light trampling, and the lowest in the level of heavy trampling. Biomass was used as a basis of the assessment of plant resistance to trampling. Three tussock species,Eragrostis tenuifolia, Lolium perenne, andSporobolus elongatus had a high resistance. Woody and erect herbaceous plants were more intolerant to trampling. There appear to be two processes involved in the reduction of the plant parameters: direct physical damage with portions of the plants detached, and physiological changes, which slow down vegetative growth rates. Plant height was found to be the most sensitive indicator of trampling damage.     

Scale-dependent inconsistencies in the effects of trampling on a forest understory community

The response of forest understory vegetation to trampling applied at different temporal and spatial scales was determined in a cliff-edge forest in Ontario, Canada. Three frequencies (0, 50, 500 passes per year) of short-term trampling (one year) were applied to plots previously undisturbed. Existing trails that had received three frequencies (approx. 100, 500, 25,000 passes per year) of long-term trampling (18 years) were also studied. Community composition, species richness, and individual species frequency were recorded in plots within 4 m and (or) 1 m of the patch centerline. The quantitative and qualitative form of plant response to increased trampling was compared for short-term and long-term treatments, both within 4 m and within 1 m of the path centerline, to judge the consistency of trampling effects at different temporal and spatial scales. As trampling frequency increased, community composition changed progressively, but consistently, in plots both within 4 m and 1 m of the path centerline. Species richness was less affected by trampling and only decreased within 1 m of the path centerline at the highest level of trampling (25,000 passes per season for 18 years). Effects of trampling on individual species frequency were much less consistent at different temporal and spatial scales of trampling. The scale-dependence results suggest that field workers and resource managers both should try explicitly to include and define multiple scale components when trying to ascertain the response of vegetation to human disturbance factors.     

Responses of Three Heathland Shrubs to Single or Repeated Experimental Trampling

In the last few decades, trampling has become a cause of major disturbance to natural areas. Trampling can occur regularly either seasonally or throughout the year, or as a single impact depending on a recreational activity or an event. This study, carried out on a coastal heathland in north Brittany (France), was based on the hypothesis that the vegetation response can differ depending on whether the trampling is repeated or due to a single event. The aim was to compare the resistance and the short-term recovery capacity of selected species in coastal heathland communities subjected to a trampling experiment, applied once or five times (with equal total number of passes from 0 to 800). The temporal distribution of the trampling did not affect the response of Erica ciliaris, whereas for Erica cinerea and Ulex gallii, trampling was slightly less damaging when applied once rather than five times. These differences could be linked to the lower resistance and the higher recovery capacity of E. ciliaris, compared with the two other species. This study also emphasized the influence of site location by comparing observations with previous studies. Plant resistance calculated here appeared to be lower than that calculated for a less stressful site. The relationship between plant traits and response to trampling was also investigated. The effects of the different times between disturbance application and observations of the short-term response of woody vegetation to experimental trampling is also discussed.     

The wilderness use simulation model applied to Colorado River boating in Grand Canyon National Park,USA

A modification of the Shechter-Lucas Wilderness Use Simulation Model (WUSM) for peak season boating on the Colorado River through Grand Canyon National Park, USA, is evaluated as a tool for making management decisions. A new microcomputer program to select trip itineraries for inclusion in the WUSM that was developed as part of this study is presented. This program simplifies user input and expands the WUSM's usefulness as a tool for management decisions by randomizing itinerary schedules based on probabilities developed from actual use of sites by canyon visitors. Model usefulness is demonstrated by simulating various management changes and comparing use levels of attraction sites and campgrounds as well as numbers of encounters between parties. The WUSM is being used as part of an ongoing study, to reflect the impact of fluctuating flow regimes through the turbines at Glen Canyon Dam on river trips.     

Impacts of experimental trampling on tall alpine herbfields and subalpine grasslands in the Australian Alps

The Australian Alps, which are of high conservation value, are popular summer bushwalking destinations. Experimental trampling trials using a standardized methodology were conducted to determine the resistance and resilience of the two common vegetation types: tall alpine herbfield and subalpine grasslands. Vegetation parameters were measured in lanes subject to control (no trampling), 30, 100, 200, 500 and 700 passes at five sites prior to trampling, immediately post trampling, 2 weeks, 6 weeks and 1 year post trampling. Vegetation height, cover of graminoids and herbs, as well as net species richness all declined with trampling, while litter cover increased. Thresholds for damage varied between the two communities and among the different vegetation parameters. The resistance indices for the two communities (number of passes resulting in 50% reduction in vegetation cover), however, were similar at around 440–450 passes indicating that these two communities are among the most resistant of the 19 alpine–subalpine communities that have been tested around the world, but only of moderate resistance compared to non-alpine communities. The two communities showed limited recovery with damage still evident 1 year post trampling. This indicates that they have only moderate tolerance to damage due to moderate resistance, but low resilience.     

The response of two arctic tundra plant communities to human trampling disturbance

A 4-year study was conducted to evaluate the consequences of human trampling on dryas and tussock tundra plant communities. Treatments of 25, 75, 200 and 500 trampling passes were applied in 0.75 m2 vegetation plots at a time of approximately peak seasonal biomass. Immediately after and 1 and 4 years after trampling, plots were evaluated on the basis of plant species cover, percent bare ground, vegetation height, and soil penetration resistance. One year after trampling, soils were collected for nitrogen analysis in highly disturbed and control plots. Immediately after trampling, 500 trampling passes resulted in approximately 50% cover loss in the dryas tundra and 70% cover loss in tussock tundra, but both communities showed a substantial capacity for regrowth. Plots where low and moderate levels of trampling were applied returned to pre-disturbance conditions by 4 years after trampling, but impact was still evident in plots subjected to high levels of disturbance. These results suggest that these tundra communities can tolerate moderate levels of hiking and camping provided that use is maintained below disturbance thresholds and that visitors employ appropriate minimum-impact techniques. By utilizing this information in a visitor education program combined with impact monitoring and management, it is possible to allow dispersed camping and still maintain these vegetation communities with a minimum of observable impact.     

LOS ALAMOS FOREST FIRE IMPACT MODELING1

ABSTRACT: A controlled burn at Bandelier National Monument got out of control and burned about 43,000 acres (17,400 hectares) near Los Alamos, New Mexico, in May 2000. The wildfire caused dramatic changes in infiltration capacity and wettability of soils in many of the watersheds above Los Alamos National Laboratory (LANL) and destroyed the duff layer, dramatically reducing the interception and infiltration capacity of the formerly forested watersheds. These sudden changes in basin hydrology necessitated a rapid assessment of hydrology and hydraulics for the canyons running through LANL property to evaluate flood risk, plan emergency flood protection measures, and assess potential sediment and actinide transport. This paper presents the results of hydrologic and hydraulic modeling of Los Alamos Canyon following the wildfire. The large scale modeling effort, with over 13,000 cross sections for the hydraulic model (5,000 for Los Alamos Canyon, 8,000 for Guaje Canyon), relied heavily on a geographic information system (GIS) for model input and floodplain delineation. The HEC‐geoRAS model provided good integration between the hydraulic model (HEC‐RAS, Version 3.0.1) and the GIS (ArcView, v. 3.3). These modeling results are being used in drainage master planning efforts at LANL and in the development of sediment transport models using HEC‐6T. Sediment transport modeling results will be used to develop actinide transport models for the canyons at LANL.     

Impacts after four years of experimental trampling on alpine/sub-alpine environments in western Tasmania

Experimental trials were undertaken over four years to assess the impact of recreational trampling in undisturbed alpine and sub-alpine vegetation communities in the Western Arthur Range, western Tasmania. Data on 'pad' formation due to human trampling were collected using vegetation cover assessments, biomass estimates and detailed cross-sectional surface profiles. In sub-alpine buttongrass and alpine herbfield, prolonged and sustained damage may occur after 100 passes by walkers. The environmental threshold of the flat alpine herbfield site was breached after 200 passes. Plant morphology was one determinant of resistance and resilience, with upright woody shrubs and tall tussock graminoids most vulnerable to sustained trampling damage. Cushions are susceptible to trampling impacts at 500 passes. Loss of vegetation cover peaks 6-12 months after trampling. Our results show that pads formed with as few as 30-100 passes per annum and tracks form at between 100 and 500 passes per annum. Two years after the cessation of trampling, there is some small recovery in vegetation cover after 30 and 100 passes per annum applied for three years, but no evidence of recovery at the 500 pass treatments. The low trampling threshold and slow recovery rates in western Tasmania suggest that concentrating walkers on a minimal number of sites may be the best management option for these untracked alpine and sub-alpine environments.     

Effects of Trampling Limitation on Coastal Dune Plant Communities

Sandy coastlines are sensitive ecosystems where human activities can have considerable negative impacts. In particular, trampling by beach visitors is a disturbance that affects dune vegetation both at the species and community level. In this study we assess the effects of the limitation of human trampling on dune vegetation in a coastal protected area of Central Italy. We compare plant species diversity in two recently fenced sectors with that of an unfenced area (and therefore subject to human trampling) using rarefaction curves and a diversity/dominance approach during a two year study period. Our results indicate that limiting human trampling seems to be a key factor in driving changes in the plant diversity of dune systems. In 2007 the regression lines of species abundance as a function of rank showed steep slopes and high Y-intercept values in all sectors, indicating a comparable level of stress and dominance across the entire study site. On the contrary, in 2009 the regression lines of the two fenced sectors clearly diverge from that of the open sector, showing less steep slopes. This change in the slopes of the tendency lines, evidenced by the diversity/dominance diagrams and related to an increase in species diversity, suggests the recovery of plant communities in the two fences between 2007 and 2009. In general, plant communities subject to trampling tended to be poorer in species and less structured, since only dominant and tolerant plant species persisted. Furthermore, limiting trampling appears to have produced positive changes in the dune vegetation assemblage after a period of only two years. These results are encouraging for the management of coastal dune systems. They highlight how a simple and cost-effective management strategy, based on passive recovery conservation measures (i.e., fence building), can be a quick (1–2 years) and effective method for improving and safeguarding the diversity of dune plant communities.